1. Field of the Invention
The present invention relates to the field of extracorporeal blood circulation in a single needle circuit. More particularly, the invention relates to a method and apparatus for controlling alternating flow within a single needle circuit having arterial and venous stages.
2. Description of the Related Art
In an extracorporeal treatment, blood is drawn from the patient, treated by an appropriate device and then restored to the patient. The drawing and restoration of blood to the patient can be undertaken via two different access points on the patient's body. However, when treatments have to be carried out in a repetitive manner such as is necessary with dialysis treatments of patients with chronic renal insufficiencies, it is advantageous to limit the number of punctures and to use a single needle extracorporeal blood circuit. In a single needle circuit, blood is drawn and restored to the patient via a single vascular access point. When a single needle system is used, the blood must alternately circulate from the needle in one direction and then to the needle in an opposite direction. It is therefore necessary to resolve problems associated with the alternation of the aspiration or arterial stages and the restoration or venous stages.
U.S. Pat. No. 4,776,837 relates to a single needle extracorporeal circuit, including an arterial portion comprising an arterial reservoir and means for opening and closing of the circuit situated ahead of the arterial reservoir and downstream from the needle. It also includes a venous portion comprising a venous reservoir and means for opening and closing the circuit situated downstream of the venous reservoir and upstream of the needle.
According to U.S. Pat. No. 4,776,837, the operation of the venous stage, also referred to as the blood restoration stage, is initiated in response to the presence of a top quantity limit of liquid inside the venous reservoir. Similarly, the operation of the arterial stage, also referred to as the blood aspiration stage, is initiated in response to the presence of a bottom quantity limit of liquid inside the arterial reservoir.
The authors of the '837 patent suggest that their method for alternately operating the arterial and venous stages is self-regulating and affords an optimum use of treatment time because the arterial stage is only started when the quantity of liquid present in the arterial reservoir becomes insufficient, and the venous stage is only started when an adequate quantity of liquid is present in the venous reservoir.
However, there are safety problems with this related art control method. During the arterial stage, the quantity of liquid increases simultaneously in both the arterial and venous reservoirs. If the upper limit condition is only detected in the venous reservoir, it is possible for an unduly high quantity of liquid to be present in the arterial reservoir before the top quantity limit is reached in the venous reservoir, which can result in blood overflowing from the arterial reservoir.
Similarly, during the venous stage, the quantity of liquid diminishes simultaneously in the two reservoirs. If only the presence of a bottom quantity limit in the arterial reservoir is detected, the quantity of liquid present in the venous reservoir may become insufficient. There is then a risk that air will be injected into the patient along with treated blood, which can cause serious disorders such as embolisms.